(1) Field of the Invention
This invention relates to cone beam type X-ray CT apparatus for performing CT (computerized tomography of selected sections) by irradiating an object under examination such as a patient or an article with X rays emitted in a diverging cone form. More particularly, the invention relates to a technique for accurately determining a reconstruct area in advance of CT imaging.
(2) Description of the Related Art
X-ray CT apparatus widely used in hospitals and the like include, as shown in FIG. 1, an X-ray tube 71 for emitting a fan-shaped X-ray beam FB, and an X-ray line detector 73 with X-ray detecting elements 72 arranged in a row or a plurality of rows along a spreading direction of the X-ray beam. The X-ray tube 71 and X-ray line detector 73 are constantly opposed to each other across a patient M, and constitute an X-ray image pickup unit 74. In time of CT imaging, the X-ray tube 71 and X-ray line detector 73 revolve around the patient M, with the X-ray tube 71 emitting the fan-shaped X-ray beam FB, and the X-ray line detector 73 outputting transmitted X-ray detection data. A reconstruction process is performed to produce CT images based on the detection data. However, with X-ray CT apparatus shown in FIG. 1, since the fan-shaped X-ray beam FB has only a very small width, the X-ray image pickup unit 74 must be shifted along the body axis of patient M and revolved many times around the patient M in order to collect transmitted X-ray detection data necessary for a CT imaging covering a wide range. This inevitably consumes a long time in image pickup.
With this in view, an X-ray CT apparatus is being considered, which employs a wide conical X-ray beam instead of the fan-shaped X-ray beam FB, and a flat panel X-ray sensor (X-ray area sensor) instead of the X-ray line detector 73, the flat panel X-ray sensor having a large detection surface with X-ray detecting elements arranged in a matrix form. While the conical X-ray beam irradiates a site of interest of an object under examination all at once, the flat panel X-ray sensor having the large detection surface detects X rays transmitted through the site of interest all at once. This cone beam type X-ray CT apparatus can acquire transmitted X-ray detection data covering a wide range, with one rotation about the patient M of the X-ray image pickup unit. This results in a substantial reduction in image pickup time.
However, compared with the fan beam type X-ray CT apparatus, the conical beam type X-ray CT apparatus has a disadvantage that it is difficult to determine a reconstruct area in advance of CT imaging.
With CT imaging, overlapping portions of all X-ray beams emitted from around the body axis of patient M provide a reconstruct area, i.e. an imaging range, for making CT images of selected sections. It is therefore necessary, before starting an image pickup operation, to determine a reconstruct area carefully and confirm that a site of interest to be imaged is included in the reconstruct area. With the conical beam type X-ray CT apparatus, however, since it is difficult to determine a reconstruct area beforehand, an inconvenience could occur after an image pickup operation that CT images acquired fail to show desired sections, with the site of interest excluded from the reconstruct area.
Further, in a conventional practice of acquiring sectional images of patient M, a so-called scanning plan is made for a particular site of interest by setting an imaging range, positions from which sectional images are to be picked up, and sectional image pickup intervals of 10 mm, for example, based on a fluoroscopic image of patient M acquired beforehand. Based on the scanning plan, a relative position between the patient M and the apparatus is determined to be suitable for an image pickup starting position, and then the apparatus is operated.
In the conventional practice, however, while the scanning plan is based on a fluoroscopic image of patient M, an actual image pickup starting position is determined with reference erence to the position of patient M at that time rather than the fluoroscopic image. Consequently, an error in the position of patient M relative to the apparatus would result in a deviation of the imaging range, making desired sectional images impossible.
Furthermore, depending on patients M, diverse sites of interest may have to be imaged. These sites include, for example, one having a large imaging range transversely of the body, such as the whole stomach or the whole abdomen, one having a small imaging range transversely of the body but a large imaging range along the body axis, such as the gullet or the vertebra, and one having small imaging ranges both transversely of the body and along the body axis, such as the liver.
To cope with the varied situations, the X-ray CT apparatus includes an aperture or the like for the X-ray source to vary the size of a reconstruct area of X rays irradiating the patient M.
Conventionally, however, a reconstruct area of X rays is determined unequivocally according to an image pickup site when making a scanning plan. Particularly where the reconstruct area is small, an error tends to occur in the relative position between the reconstruct area of X rays and the patient M in time of actual image pickup, making desired sectional images impossible to acquire.
This invention has been made having regard to the state of the art noted above, and its primary object is to provide a conical beam type X-ray CT apparatus for accurately determining a reconstruct area in advance of CT imaging.
Another object of this invention is to allow desired sectional images to be acquired properly by correctly setting, to an object under examination, an imaging range and positions of sectional images to be reconstructed, based on a fluoroscopic image picked up of the object in time of actual imaging.
To fulfill the above objects, this invention provides the following constructions:
(1) An X-ray CT apparatus for picking up sectional images of an object by irradiating the object with a conical X-ray beam from around a body axis of the object and detecting X rays transmitted through the object, the apparatus comprising:
an X-ray image pickup device including an X-ray tube for emitting the conical X-ray beam to the object placed on a top board, and a flat panel X-ray sensor with X-ray detecting elements arranged in a matrix form for detecting transmitted X rays;
a drive device for driving the X-ray image pickup device about the body axis of the object;
an image reconstruct device for reconstructing a sectional image of a designated section based on transmitted X-ray detection data outputted from the flat panel X-ray sensor of the X-ray image pickup device driven in a fluoroscopic imaging mode;
an image display device for displaying X-ray images;
a fluoroscopic imaging device for acquiring a fluoroscopic image based on the transmitted X-ray detection data outputted from the flat panel X-ray sensor of the X-ray image pickup device driven in the fluoroscopic imaging mode, and displaying the fluoroscopic image on the image display device; and
a reconstruct area superimposing device for displaying a reconstruct area for CT imaging as superimposed on the fluoroscopic image acquired by the fluoroscopic imaging device.
With the above apparatus according to the invention, a fluoroscopic imaging (simple X-ray imaging) operation is carried out first to acquire a fluoroscopic image for confirming a reconstruct area. That is, the X-ray image pickup device is set to an appropriate position for the fluoroscopic imaging mode, and the object is irradiated with a conical X-ray beam from the X-ray tube. The flat panel X-ray sensor detects a transmitted X-ray image. Based on the transmitted X-ray detection data outputted from the X-ray sensor, the fluoroscopic imaging device acquires a fluoroscopic image and displays it on the image display device. At this time, with the X-ray CT apparatus according to this invention, the reconstruct area superimposing device simultaneously displays a reconstruct area for CT imaging on the fluoroscopic image (i.e. superimposed display). Only an area where all conical X-ray beams emitted from around the body axis of the object overlap one another provides the reconstruct area (i.e. an imaging range). The reconstruct area corresponds to an entire inner area of a circle around the center (imaging center) of a gantry and inscribed in the conical X-ray beam. The reconstruct area, when superimposed on the fluoroscopic image, appears in a circular shape. That is, before starting the CT imaging, the operator may check whether or not a site of interest to be imaged is within the reconstruct area superimposed on the fluoroscopic image. The operator starts the CT imaging after confirming that the site of interest is within the reconstruct area. As a result, CT images of desired sections may be obtained reliably. In the CT imaging by the apparatus according to this invention, the site of interest to be imaged is irradiated with wide conical X-ray beams all at once, and X rays transmitted through the site of interest are detected all at once by a large detection surface of the flat panel X-ray sensor. Only a short imaging time is required since transmitted mitted X-ray detection data covering a wide range is collected quickly.
(2) Preferably, the apparatus (1) above further comprises a slice designating device for designating positions of sections for CT imaging on the fluoroscopic image with the reconstruct area superimposed thereon. With this construction, positions of sections of which CT images are to be made may be designated on the fluoroscopic image with the reconstruct area superimposed thereon to indicate a range for designating positions of sections. The operator can properly and simply designate positions of sections of which CT images are to be made by using the reconstruct area as a guide.
(3) Preferably, the apparatus (1) above further comprises prises an irradiating area superimposing device for displaying an area irradiated by the conical X-ray beam, as superimposed on the fluoroscopic image. With this construction, an area of the object irradiated by the conical X-ray beam from the X-ray tube may be displayed as superimposed on the fluoroscopic image. Thus, the operator may confirm an area of the object to be exposed to X rays.
(4) Preferably, the apparatus (1) above further comprises prises a moving device for moving the object along the body axis of the object relative to the X-ray image pickup device. With this construction, a fluoroscopic image may be acquired from a selected position along the body axis of the object.
(5) Preferably, the apparatus (4) above further comprises prises a fluoroscopic image composing device for composing and outputting one fluoroscopic image based on transmitted X-ray data corresponding to a plurality of images of the object acquired by driving the moving device to move the object. With this construction, where the site of interest of the object cannot fit into the irradiating area of the conical X-ray beam, a fluoroscopic image may be obtained in the fluoroscopic imaging mode from each of several overlapping or adjacent positions covering the site of interest. Main portions of a plurality of fluoroscopic images obtained as above may be combined to form one fluoroscopic image for display on the image display device. That is, the operator may carry out an operation based on one fluoroscopic image, without having to switch the image display device and the like frequently, which realizes an increased efficiency of operation.
(6) Preferably, the apparatus (4) above further comprises prises an imaging range setting device for setting an imaging range on the fluoroscopic image displayed on the display device, wherein the moving device is operable to move the object over the imaging range set by the imaging range setting device, and the slice designating device is operable to designate positions of sections within the imaging range.
With this construction, while confirming the reconstruct area displayed on the display device, the operator may designate positions of sections to be imaged, to acquire CT images in a desired range reliably.
(7) Preferably, the apparatus (6) above further comprises prises an irradiating field setting device for setting a field size of X rays to irradiate the object according to the reconstruct area. With this construction, the site of interest of the object to be imaged may be irradiated with a conical X-ray beam of a size suited to the size of the site of interest.
(8) Preferably, in the apparatus (7) above, the irradiating ating field setting device includes a collimator and an aperture drive device for setting a field size of the conical X-ray beam emitted from the X-ray tube. With this construction, the irradiating area of the X-ray beam for irradiating the object and the reconstruct area may be controlled easily by the operator.
(9) Preferably, the apparatus (8) above further comprises prises a scan mode selecting device, operable based on the field size of X rays set by the irradiating field setting device and the imaging range set by the imaging range setting device, to select a driving mode such that a single scan is carried out by driving the drive device with the moving device stopped when the field size along an axis of revolution of the X-ray image pickup device is larger than the imaging range, and a spiral scan is carried out by simultaneously driving the drive device and the moving device when the field size along the axis of revolution of the X-ray image pickup device is smaller than the imaging range.
With this apparatus, after setting a size of the reconstruct area of X rays emitted in a diverging conical form to the object and an imaging range of the object, the single scan or spiral scan is automatically selected based on a relationship between the size of the reconstruct area along the axis of revolution of the X-ray source and the imaging range, so as to cover the entire imaging range. Thus, sectional images may be picked up easily and effectively with minimal exposure to X rays regardless of the patient""s physique, the size of the article and the size of the site of interest to be imaged. Particularly where the object is a patient, X-ray irradiation over a larger area than necessary may be avoided to provide an advantage of reducing the burden imposed on the patient.